Conventionally, an adaptive cruise control (ACC) radar has been used in the case where a vehicle follows a forward vehicle with a predetermined distance therebetween as illustrated in FIG. 1. However, the requirements for the radar device have been increased. Thus, when the forward vehicle suddenly stops so that the interval between the vehicles momentarily decreases, the target may not be easily detected only through the radar device. FIG. 2 is a graph illustrating a transmit (Tx) signal and a receive (Rx) signal of an FMCW radar device. Theoretically, only a frequency difference between the Tx signal and the Rx signal must be extracted as illustrated in FIG. 3. Due to the characteristic of the radar, however, the radar directly mixes the Tx signal and the Rx signal to extract the best signal. Thus, a large DC component may exist as illustrated in FIGS. 4 and 5. In the case of a short-distance object as illustrated in FIG. 4, however, when the frequency is moved toward the DC component region due to a speed difference, the signal cannot be extracted. In this case, the radar may miss the target. That is, the control distance of the radar may be limited.
Furthermore, when an in-phase (I) channel and a quadrature (Q) channel exist for each channel in a receiver circuit of the radar device, the sign of speed may be extracted through phase information. However, when I/Q channels do not exist for each channel, the sign of frequency is extracted as only a positive value corresponding to an absolute value as illustrated in FIG. 3. In this case, since phase components are not detected in case of short distance and high speed, there exists a frequency region in which the target cannot be detected.
(Patent Document 1) Korean Patent No. 1,083,660 registered on Nov.9, 2011.